Summary:
There are two distinctly different types of basin successions; one forming linear geometries such as the Mt Lucky Conglomerate, and a second type; the Wallaby Conglomerate, that wraps around the southern margin of the Mt Margaret Dome.
Mineralisation
Granny Smith corridor
The Granny Smith gold deposits (including the Windich, Granny and Goanna lodes) occur along a north-trending structural corridor. The eastern contact zone of a granitoid intrusion within the metasedimentary rocks is the locus of the Granny Smith corridor mineralisation. The metasedimentary rocks consist of quartz-rich greywacke, lithic wacke and minor shale, and sandstone interbeds, with a basal conglomerate of clasts of shale, quartz, quartzite, banded iron formation (BIF) and feldspar-phyric volcanic rocks.
The Granny Smith granodiorite is a small (2 km × 5 km) elongate pluton which is porphyritic and has mafic-rich margins. Aplitic pegmatite dykes, which only cut the granitoid, represent the last magmatic phase. The present erosional level is close to the roof of the pluton, and in places has a thin cap of sedimentary rocks.
The pluton is surrounded by a 200 m-300 m wide contact metamorphic aureole that is progressively zoned from the granitoid contact outwards through a several metres-wide hornfelsed margin, to an andalusite-bearing slate, and finally to a slate with mica spots. Intrusive breccias and miarolitic cavities suggest a high-level intrusion, where a volatile phase was exsolved from the magma during or after high-level emplacement. The granitoid and gold mineralisation are cut by carbonatite and lamprophyre dykes. Late-stage plagioclase porphyry dykes have also intruded into the sedimentary sequence.
Wallaby
The Wallaby gold deposit is hosted within a thick matrix-supported, mafic conglomerate unit (Wallaby conglomerate) that dips moderately to the southeast and is intruded by a suite of fractionated alkaline dykes on the south-eastern margin of the Mt Margaret Dome. Regional-scale structures near the deposit include:
• Dominant north-south trending, near vertical fault systems such as the Chatterbox Fault.
• East-northeast trending faults, dipping moderately to steeply to the south.
The Chatterbox Fault is hypothesized as one of the main structures that control the location of the Wallaby deposit. The east-northeast faults are interpreted as extensional faults developed during basin growth that were later inverted. One of the largest of these structures, Thet’s Shear, is a 250 m wide zone of intense alteration and ductile deformation that underlies the Wallaby deposit.
The alkaline dyke suite in the core of the Wallaby gold deposit displays increasing fractionation from an early-stage northeast trending mafic monzonite through to monzonite, followed by syenite, syenite porphyry, and late-stage carbonatite. The intrusives are concave in profile, stacked one above the other, plunge 50 º to the south and are inferred related to the intersection of north-south and northeast trending structures. Late-stage post-mineralisation lamprophyres occur as narrow east-northeast trending vertical dykes.
A 600 m × 800 m wide zone consisting of actinolite-magnetite-calcite ± epidote ± pyrite alteration affects the intrusives and conglomerates and forms a broadly pipe-shaped body dipping approximately 50 º to the south. Early magmatic intrusions such as mafic monzonite and monzonite are overprinted by the actinolite-magnetite-calcite alteration, while younger syenite and porphyritic syenite intrusions are unaltered. This suggests that some intrusions predate or are synchronous with the main alteration event.
The distal alteration assemblages comprise chlorite, calcite and magnetite, with the magnetite content decreasing with distance from the centre of the pipe.
Four dominant gold-related structural domains are identified in the Wallaby deposit:
• Fracture mesh lodes developed at the intersections of brittle structures (faults and veins), dip and strike changes along structures, and gold mineralisation linked to the development of laminated and quartz breccia veins.
• Horsetail domain lodes developed where the lode fault diverges into multiple faults with extensive extension veins formed between the faults.
• Ductile shear-controlled gold lodes resulting from the intersection between brittle structures and earlier ductile shears and cleavages associated with north oriented shearing.
• Hematite associated gold lodes distinguished by an assemblage of quartz, iron-rich dolomite, hematite and gold.
The gold lodes form a series of stacked zones with two distinct geometries; 5 to 20 m thick sub-horizontal to gently northeast dipping lodes measuring approximately 800 m along strike and 1.1 km down dip and narrower, moderately northeast-dipping lodes. The major flat lying lodes typically have vertical separation of 150-250 m and offset to the south with depth, following the hanging-wall of Thet’s shear zone. Existing models suggest that the low angle lodes formed within a north-west/south-east phase of compression. A series of east-west striking vertical lodes is present as linking structures between the main horizontal lodes.
The main gold mineralising event is associated with dolomite-albite alteration which bleaches the conglomerate to a distinctive fawn/green colour. Sulphidation appears to be the main gold deposition mechanism. Lower grade areas are associated with carbonate-biotite and early-stage hematite alteration.
Gold occurs along micro-fractures within pyrite and generally ranges from 1 -20µm in size. It also occurs as larger (up to several millimetres) grains within quartz veining, either in strongly dolomite-albite altered rocks or in small veins within a few metres of an ore zone. Coarse visible gold is also observed in narrow (<5 mm) moderate to steeply dipping quartz-carbonate veins.
Hillside
The geology of the Hillside gold deposit, 6 km south-west of Granny Smith, comprises three zones separated by two interpreted north-northeast trending first order faults of the Laverton Tectonic Zone: • Western zone - mafic volcanic rocks, BIF, argillite and arenite with irregularly developed polymictic conglomerate.
• Central zone – basalt, ultramafic flows, argillites, BIF and minor dolerite.
• Eastern zone - shale and turbiditic sequence intruded by dolerite. The area has undergone metamorphism to greenschist facies.
Gold mineralisation at Hillside is hosted by a carbonate-muscovite-quartz-pyrite-(magnetite-biotite) altered basalt exhibiting a pervasive shear fabric. There is frequent segregation into carbonate-quartz-rich and muscovite-rich domains, leading to a banded appearance. Mineralisation occurs along a strike length of 480 m and extends 200 m down dip.
Deposit types
Archaean orogenic lode mineralisation is the primary source of gold at Granny Smith.
The Wallaby lodes are flat-lying alteration zones hosted in magnetite amphibole altered conglomerates. Mineralised zones show moderate to long-range geological continuity and short-range grade continuity.
Other deposits and targets comprise of vein stock works and shears.